A lubricant that includes an oil-in-water emulsion (O/W emulsion), in more detail a high-viscosity lubricant that includes a thickener that may stabilize an aqueous composition and an oil-in-water emulsion, and a method of producing the same have been known (see Patent Document 1, for example). Patent Document 1 discloses a lubricant that includes an oil-in-water emulsion having a certain viscosity measured using a Brookfield viscometer, wherein water as the main component includes a water-insoluble oil-soluble EP agent (extreme pressure agent), and a water-soluble liquid organic dispersant that dissolves the EP agent and stably disperses the EP agent in water, and the oil of which discontinuous dispersion phase is formed is a synthetic oil, and a method of producing the same. The main components of the patent, that is, sulfur, chloro-sulphur, chlorinated aliphatic hydrocarbon, and phosphorus EP agent produce corrosion products, such as sulfide, chloride, phosphide and the like on a sliding friction surface to form a solid lubricant layer on the surface, and thus have a function to improve the lubrication properties. Patent Document 1 discloses that the above configuration achieves excellent lubrication properties as compared with EP agent-containing mineral oil which is a component of the invention or an aqueous composition in which the EP agent is stably dispersed in the presence of a grease and a water-soluble liquid organic dispersant. Patent Document 1 also discloses that the lubricant may include a small amount of solid lubricant selected from graphite, molybdenum disulfide, and a polytetrafluoroethylene powder as a corrosion inhibitor, friction modifier (friction reducing agent), film-forming agent or the like in order to further compensate for the lubrication effect of the EP agent.
A inventor of the present application have developed a method of producing an ultrafine diamond particle dispersant that may be usable to produce a lubricant, and disclose the ultrafine diamond particle dispersant which includes a dispersion medium that form an emulsion-type solvent and the form thereof is slurry or paste-type (see Patent Document 2).
The term “ultrafine diamond particles” used herein refers to ultrafine diamond particles produced by the detonation technique, fine diamond particles with an average particle size of 100 nm or less produced conventionally by a static ultrahigh-pressure method or a vapor phase synthesis method, and a mixture of ultrafine diamond particles or fine diamond particles and non-diamond or quasi-diamond (amorphous) carbon that is at least partially bonded thereto, or a mixture of the ultrafine diamond particles or fine diamond particles and isolated particulate non-diamond or quasi-diamond carbon. All of these particles and mixtures may be within the scope of the present invention unless otherwise indicated herein.
A lubricant in which ultrafine diamond particles are dispersed in a lubricating oil has been known (see Patent Documents 3, 4, 5, and 6, for example). More specifically, Patent Document 3 discloses a lubricant for rolling bearing that ultrafine diamond particles having an average particle size of 0.1 μm or less are added to a lubricating oil in a proportion of 0.05 to 15 wt % of the lubricating oil. Patent Document 4 discloses a nanoparticle-containing lubricating oil composition that contains a base oil, a hydroxyl group-containing additive, and nanodiamond particles having a particle size of 10 nm or less. Patent Document 5 discloses a lubricant composition that is produced by adding 0.01 to 1.0 mass % of a solid friction modifier to a base oil, wherein the solid friction modifier contains 2 to 99 mass % of abrasion-resistant diamond having a cluster size of 1 to 10 nm, and 1 to 98 mass % of graphite. Patent Document 6 discloses a lubricant in which ultrafine diamond particles with rounded shape and having a particle size of 10 nm or less are dispersed in a lubricating oil.
A lubricant produced by adding ultrafine diamond particles to lithium soap grease has been known (see Non-patent Document 1, for example). Non-patent Document 1 discloses that the ultrafine diamond particles improve abrasion resistance and seizure resistance based on the Falex test results.
Non-patent Document 3 discloses that the lubrication properties of a rolling oil have a correlation with the thickness of an oil film (i.e., the plate-out behaviors (i.e., the emulsion breaks on the working surface so that the surface is wetted only with the oil) have a correlation with the friction coefficient, and improve the lubricating performance). Specifically, Non-patent Document 3 discloses that excellent lubrication properties are obtained in the order of the emulsion configuration of (O/W)<(W/O)<(W/O/W), and it is desirable that the final emulsion configuration on the working surface be a W/O emulsion configuration.
A metal slide member which is provided a layer containing molybdenum disulfide which is a solid lubricant in a surface layer having a depth of 20 μm or less by causing a fine molybdenum disulfide powder to collide with the surface of the metal slide member, a surface treating method therefor, and an injection material thereof have been disclosed (see Patent Documents 7 and 8). A composition for multilayer lubricating film that may form a dry film that exhibits excellent sliding properties and adhesion to a substrate, such as piston skirt or the like, a multilayer lubricating film, and a piston having the film have been disclosed (see Patent Document 9).